Wireless and like receiver



June 21, 1938. F. T. LETT WIRELESS AND LIKE RECEIVER Filed July 12, 1955 E. 1 4 I H M W m. 6 3 W m 0 awe m M Patented June 21, 1938 UNITED STATES- 2,120,984 WIRELESS AND LIKE RECEIVER Frederic Tom Lett, Iver, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application July 12, 1935, Serial No. 30,975

I In Great Britain July 17, 1934 3 Claims.

The present invention relates to wireless and like receivers, and more particularly but not exclusively to receivers embodying automatic gain control.

In receivers embodying automatic gain control, there are usually provided a high frequency amplifier, a signal detector, and controlling means operating automatically to control the gain or amplification 0f the high frequency amplifier in such manner that, whatever the received signal strength may be within predetermined limits, a signal of substantially the same strength is passed to the signal detector.

A disadvantage of receivers of this kind is that over those parts of the tuning range of the receiver at which no signal is tuned in, the high frequency amplifier is in its most sensitive condition (that is, it has its maximum gain) and disturbances such as valve noises and the like may be amplified to such an extent as to prove objectionable. For the purpose of avoiding this disadvantage, it has been proposed to provide means for muting the receiver when no signal is being received; the muting means have in some cases been associated with frequency selective means so arranged that the receiver is only alive when a signal is accurately tuned in. With such arrangements, the receiver is quiet when no signal is being received, but in tuning the receiver from one'end of the wave band over which it is responsive to the other end thereof, the set becomes alive each time a signal is tuned in, and the operation of tuning is thus still a noisy one. v

It is an object of the present invention to provide an improved receiver in which tuning noises are further reduced.

According to the present invention, a wireless receiver, which may embody an automatic gain control, is provided with means arranged to mute the receiver when no signal is being received, and to render the receiver alive only when it is allowed to remain tuned to a desired signal for a substantial period of time, for example one or more seconds.

According to a feature of the invention, in a wireless receiver, which may embody an automatic gain control, there are provided means for biasing the control grid of a valve. forming a part of said receiver to such an extent in the negative sense relatively to the cathode thereof that said valve is substantially prevented from amplifying, andauxiliary means operating. auto matically when a signal is received to reduce the negative bias applied to said valve, the arrangement being characterized in that said auxiliary means are coupled to the control grid-cathode circuit of said valve through a circuit having .a relatively longtime constant, for example one or more seconds.

The auxiliary means may take the form of a thermionic valve having a resistance in its anode circuit, and having its anode current controlled in accordance with the strength of the received signal by means of a suitable rectifier and load resistance associated with its control grid circuit. The signal is preferably fed to thisrectifier from the output circuit of a high frequency amplifier through a circuit of very low damping so that the auxiliary means are substantially only affected by the received signal carrier.

Preferably also, the receiver is provided with a tuning indicator, which may take any known or suitable form, but which preferably comprises a milliammeter associated either with the high frequency amplifier or signal detector, or with a valve forming part of the auxiliary means.

In operation, the receiver is tuned to the wanted signal, preferably with the assistance of a tuning indicator, and the receiver is substantially completely mute throughout the whole tuning operation, although it may be momentarily in tune with several unwanted signals. Only after the receiver has been allowed to remain in tune for the predetermined period is the wanted signal heard.

A particular embodiment of the invention will now be described by Way of example with reference to the accompanying diagrammatic drawing in which Figs. 1 and 2 together show a part of a superheterodyne receiver constituted in accordance with the invention. It is to be understood that the invention is not limited to the particular embodiment described. 7

Referring to Fig. 1, an aerial-earth system A is connected to the input circuit of a high frequency amplifier which is coupled through a frequency changer to a first intermediate frequency amplifier; the two amplifiers: and the frequency changer are indicated by B. The apparatus B derives anode current from a battery 3 (Fig. 2) for which purpose terminal C of Fig. 1 is connected to terminal C of Fig. 2. Similarly terminals D and D and E and 28 are connected together. Referring to Fig. 2, the anode of the last valve V1 of the intermediate frequency amplifier of the receiver is connected through two coils l and 2 to the positiveterminal 3 of the source of anode current 3 indicted generally by a battery, the

negative terminal of which is earthed. The coils I and 2 are shunted by a condenser 4, and the circuit I, 2, 4 is tuned to the intermediate frequency. Coupled to the coil I is a further coil 5 shunted by a condenser 6, the circuit 5, 6 being also tuned to the intermediate frequency.

The intermediate frequency potential differences set up across circuit 5, 6 are applied between the anode 1 and the cathode 8 of a diode rectifier which forms a part of diode-triode valve V2. A load resistance 9 is connected between the circuit 5, 6 and the cathode 8 of the valve V2, the resistance 9 being shunted by a. condenser [8 which serves as a by-pass for currents of the intermediate frequency. The diode I, 8 constitutes the second detector of the receiver, and modulation frequency potential differences are set up across resistance 9.

An adjustable tapping point in resistance 9 is connected through a condenser II to the control grid of the valve V2, the anode of which is connected to the positive terminal 3 of the anode current source 3 through a resistance l2. The valve V2 functions as a low frequency amplifier, and is coupled through a condenser 13 to the input circuit of a power output stage P. S. which feeds loudspeaker L. S. The cathode B of the valve V2 is connected through a biasing resistance l4 shunted by a by-pass condenser 15 to a point I 6 in a potential divider constituted by three resistances l1, l8 and 19 connected in series across the anode current source. Resistance I9 is shunted by a decoupling condenser 20, and a further decoupling condenser 2! is connected in shunt with the resistances l8 and I9.

The junction point of the coils I and 2 is connected through a condenser 22 to one auxiliary anode 23 of a double diode-triode valve V3, the cathode 24 of which is connected to the junction point of the resistances I8 and IS. The anode 23 is connected to earth through vtwo resistances 25 and 26 in series, the latter resist' ance being shunted by an audio-frequency'bypass condenser 21.

The diode 23, 24, serves to provide a gain control potential difference, oscillations at the intermediate frequency established across coil 2 being applied to the diode through condenser 22. For signals abovea certain strength, current flows in the diode, and'there is set up across the resistance 26, that is, between the point 28 and earth, a potential difference which is proportional to the strength of the signal applied to the diode. The potential of the point 28 relative to earth becomes more negative as this signal strength increases, and the point 28 is conductively connected to terminal E and thus to the control grids of one or more of the high frequency and/or intermediate frequency amplifying valves, the gain of which is to be controlled. The point 28 may be connected also to the control grid of the frequency changing valve. The controlled valves are preferably of the variablemu type, and have their cathodes conductively connected to earth.

The cathode 24 of the diode is maintained at a positive potential relative to earth by the flow of current through resistance l9, and no current flows in the diode until the potential of the anode 23 exceeds that of the cathode; no automatic gain control potential difference is therefore set up across resistance 26 if the received signal strength is belowa pre-determined value depend: ent upon the potential difference across resistance 19.

Loosely coupled to the coil 2 is a resonant circuit comprising a coil 29 tuned by a condenser 30 to the intermediate frequency. One end of the circuit 29, 3D is connected to the second auxiliary anode 3| of the valve V3 and the other end is connected to the cathode thereof through a circuit comprising a load resistance 32 shunted by an audio-frequency by-pass condenser 33. The control grid of the valve V3 is directly connected to the end of the load resistance 32 remote from the cathode 24, and the main anode 34 thereof is connected through a resistance 35 and a milliammeter 36 in series to the point It. The resistance 35 and the milliammeter 36 are shunted by a bypass condenser 31, and the anode 34 of the valve V3 is connected through two resistances 38 and to the grid of the valve V2. The junction point of the two resistances 38 and 40 is connected to the cathode 24 of valve V2 through a condenser 39.

The operation of the circuit is as follows:

In the absence of a signal, the anode current of the valve V3 fiOWlng through resistance 35 biases the control grid of the valve V2 with respect to the cathode thereof to such an extent in the negative sense that substantially no anode current flows in the valve V2, which is accordingly prevented from amplifying any noise which may be applied to it, and the receiver is mute.

When a signal is accurately tuned in, current flows in the diode 3|, 24 and the sense of the potential difference set up across the load resistance 32, and applied between the control grid and cathode of the valve V2 is such as to reduce the anode current of that valve, and to make the potential of its anode more positive; 8 the triode portion of the valve V3 thus amplifies the potential difference established across the resistance 32. As the signal is tuned in, the anode current of the valve V3 decreases, this decrease being shown by the milliammeter 36; a visual indication that a signal has been tuned in is thus provided, and in order that the milliammeter may adequately perform this function, the anode and grid circuits of the valve V3 (that is, circuits 32, 33 and 35, 36, 37) arearranged to have time constants of the order of th second or less.

The change in potential of the anode of the valve V3 when a signal is tuned in is in such a sense as to reduce the negative bias on the control grid of the valve V2, and thus to allow it to amplify, and the change in potential of the anode of valve V3 is communicated to the control grid of valve V2 with a time delay of the order of one or more seconds.

The potential of the control grid of the valve V2 is dependent upon the state of charge of the condenser 39 which, as far ,as changes in potential of the anode of the valve V2 are concerned, charges through the resistance 38. The condenser 39 and the resistance 38 are accordingly given relatively large values; for example, the condenser 39 may have a capacity of 1.0 microfarad, and the resistance 38 may be of 2.0 megohms. In this case, the time lag between the tuningin of a signal and hearing the signal in the loudspeaker is of the order of two seconds.

It may be arranged that when a signal is tuned in, the .valve V3 is biased to such an extent in the negative sense thatno anode current-flows therein. In these circumstances, the control grid of thevalve V2 tends to assume the potential of the cathode thereof, and the biasing resistance I4 is therefore provided to ensure that some negative bias is at all times present.

As has been explained, for received signals havingstrengths between predetermined limits,

the effect of the automatic gain control is that "are assured since a signal of substantially constant strength is applied to the diode 3|, 24. The

values of the various circuit elements may be so chosen that unless the received signals are of greater than a predetermined strength, the valve V2 remains unable to amplify; the receiver is then permanently mute for all signals of less than said predetermined strength.

Suitable values for certain of the components I of the receiver described above (other than those to which suitable values have been assigned) have been found to be as follows:

Resistance 40 1.0 megohm Resistance 32 1.0 megohm Condenser 33 0.1 microfarad Resistance 35 0.1 megohm Condenser 3'7 0.1 microfarad It is to be understood that the invention, although described as applied to a superheterodyne receiver, is not limited to receivers of this kind.

Iclaim:

1. In a wireless receiver, a first thermionic valve for handling oscillations derived fromreceived signals, said valve having an anode, a cathode, and a control grid, a second thermionic valve having an anode, a cathode and a control grid, a resistive impedance in the anode circuit of said second valve for developing a voltage high .valve so as enough. to render the first valve inoperative in the absence of signals, a rectifier, meansfor feeding oscillations derived from received signals second valve and the control grid of said firstvalve, a resistor-condenser networlc' having a time constant of the order of one second.

2. In a signal receiving system of the type adapted to be adjusted over a band of different signal frequencies, a signal transmission tube having a signal input network and a signal reproduction output network, means connected to a gain control electrode of the tube for impressing a potential thereon of such polarity and magnitude as. substantially to reduce the gain thereof in the absence of signals of a desired amplitude, said means comprising a tube having a resistor in its space current path for developing said potential, auxiliary means, responsive to signals 'of the desired amplitude, for rendering the tube of said first means ineffective, and a coupling network connected between said resistor and said gain control electrode whose constants are chosen to impart a time delay of the order of one second to a change in said potential whereby sounds are not reproduced in said output network unless the system is adjusted to a desired signal for said time period.

3. In a signal receiving system of the type adapted to be adjusted over a band of different signal frequencies, a signal transmission tube having a signal input network and a signal reproduction output network, means connected to a gain control electrode of the tube for impressing a potential thereon of such polarity and magnitude as substantially to reduce the gain thereof in the absence of signals of a desired.

amplitude, said means comprising a tube having a resistor in its space current path for developing said potential, auxiliary means, responsive to signals of the desired amplitude, for rendering the tube of said first means ineffective, and a desired signal for said time period, a visual current indicator electrically associated with said first means to indicate the resonance condition of the system, and means electrically associated with at least the resistor of said first means to impart to it a time delay constant of the order of one-tenth second.

FREDERIC TOM LETT. 

